PROJECT SUMMARY/ABSTRACT Cone photoreceptors are the primary photoreceptor class that mediates high acuity and daylight vision. As such, their loss during the course of human disease greatly impairs visual function. Cell-based therapies to ameliorate these conditions have been proposed, in which introduction of new cones could replace those lost to disease. These cells could be produced exogenously from sources such as stem cells or from resident, in vivo populations such as Mller glia. For both strategies, it is necessary to develop new methods to specifically promote cone formation, and to assess whether de novo formed cones are molecularly equivalent to those formed during normal development. However, a significant gap exists in the current knowledge of gene regula- tory networks that promote cone photoreceptor development. The long-term goal of this project is to identify the gene regulatory networks active in cone genesis, and to devise methods based on this knowledge to gen- erate new cone cells for cell replacement therapy. Recently, transcription factors with enriched expression in early developing cones have been identified, yet the genetic requirement of these factors or how they interact with one another to promote the cone fate is unknown. The central hypothesis of this proposal is that these transcription factors are required for cone photoreceptor genesis and are part of a gene regulatory network with defined regulatory interactions. The rationale to undertake these studies is that elucidation of the normal genetic mechanisms of cone photoreceptor formation will inform the directed formation of cones from stem cells or endogenous retinal cell populations such as Muller glia. Identification of the proteins that have im- portant regulatory functions in cones will allow development of new cone induction protocols with greater and more efficacious production of functional cones. To accomplish this goal, the following three aims are pro- posed: 1) A functional analysis of early cone-enriched transcription factors, 2) Identification of key expression and functional parameters of Sall1, a transcription factor involved in cone induction and rod repression, and 3) Identification of Nrl cis-regulatory elements that repress Nrl expression in cones. The first aim will use an effi- cient gene editing and single cell transcriptomic approach to determine the necessity of newly identified tran- scription factors expressed in early stage cones. The second aim will investigate the endogenous role of Sall1 in cone formation and the extent to which expression of this gene is sufficient to induce cone transcriptional programs. The third aim will identify the transcriptional mechanisms that prevent the rod-specific transcription factor Nrl from being expressed in cones. This approach is innovative because it will investigate the early mo- lecular events in cone formation using high-resolution and robust methods. Completion of the proposed project will result in significant insights into the gene regulatory networks that promote cone formation and will inform the development of rationally designed and effective methods to promote de novo cone formation.